1. Field of the Invention
The present invention relates to a sheet feeding device that separates and conveys the uppermost sheet from a sheet stack using electrostatic attraction method, and an image forming apparatus including the sheet feeding device.
2. Discussion of the Related Art
Background sheet feeding devices that separate and convey the uppermost sheet from a sheet stack include those that separate and feed stacked sheets, such as documents and recording sheets, by using frictional force, those that separate and feed sheets by air suction.
In background sheet feeding devices using the frictional separation method, which separates sheets by using frictional force, a material such as rubber is used to form feeding rollers. Therefore, a change over time in the condition of the feeding rollers due to abrasion or the like results in a change in the frictional force exerted by the feeding rollers, that is, consequent deterioration of feeding performance. Further, when separating and feeding sheets having unequal coefficients of friction due to variations from sheet to sheet, or when separating and feeding sheets having inherently different coefficients of friction in the same feeding operation, the frictional force acting between the feeding rollers and the sheets changes. In some cases, therefore, the separation of sheets fails, or multiple feeding occurs in which a plurality of sheets are fed together. Further, the feeding rollers need to be pressed against the sheets in order to function and in some cases the sheets are dirtied or damaged as a result.
By contrast, background sheet feeding devices using the air suction method, which separates sheets by air suction, employ a non-frictional separation method not relying on the frictional force acting between the feeding rollers and the sheets, and thus the above-described problems do not arise. However, the sheet feeding device requires a blower and a duct for the air suction. As a result, the sheet feeding device is increased in size, and the sound accompanying the air suction constitutes noise. Therefore, this type of sheet feeding device is not suitable for use in an office environment.
In view of the above, as one non-frictional separation methods, an electrostatic attraction separation method has been proposed which generates an electric field in a dielectric belt and brings the dielectric belt into contact with a sheet to attract and separate the sheet from other sheets.
Specifically, a background sheet feeding device according to the electrostatic attraction separation method first applies an alternating charge to an endless dielectric belt wound around a plurality of rollers, and swings or translates the dielectric belt relative to a sheet stack such that the dielectric belt approaches or contacts the sheet stack. Then, the sheet feeding device causes the dielectric belt to stand by for a predetermined time to attract the uppermost sheet of the sheet stack, and thereafter moves the dielectric belt away from the sheet stack, thereby, separating the uppermost sheet and conveying it from the sheet stack.
The sheet feeding device using the electrostatic attraction separation method is advantageous in preventing not only the abrasion of the feeding rollers and the damage to the sheets, which occur in the frictional separation method, but also the increase in device size and the noise generation, which occur in the air suction method.
When separating and feeding relatively thick sheets or sheets difficult to attract due to the electrical characteristics thereof, however, sheet feeding devices using the electrostatic attraction separation method need to extend the predetermined time for causing the dielectric belt to stand by to have the attraction force thereof act on the uppermost sheet. As a result, the productivity suffers.